The playback principles of the present invention are applicable to the recovery of data recorded in an information track as a succession of undulations of varying length along the length of the track.
In certain high density information playback systems, video information is recorded as relatively short wavelength (e.g., 0.4 .mu.m relief variations along the length of an information track. Illustratively, the method of recording may be of the type shown in U.S. Pat. No. 4,044,379, issued to J. B. Halter. Pursuant to the Halter method, an electromechanically driven stylus (e.g., of diamond) responsive to a combined video and audio signal, records relatively short geometric variations representative of the time variations of the signal in a metal substrate. After the electromechanical recording operation, the metal substrate has a relief pattern corresponding to that which is desired in the final record. Stampers which are used to produce production line records are made from the substrate and a vinyl record is formed, having the desired relief pattern, from the stamper.
In one illustrative format for electromechanical cutting disclosed in the Halter patent, an encoded video signal is additively combined with the accompanying encoded audio signal. In accordance with this method, the accompanying encoded audio signal is obtained by causing the audio signal to frequency modulate a low frequency sound carrier over a low frequency deviation range (illustratively, 716 .+-.50 KHz). The encoded video signal is obtained from a picture modulator, wherein the composite color video signal (including luminance signals occupying a given band of frequencies and chrominance signals appearing as sideband components of a modulated chrominance subcarrier interleaved with the luminance signal components in an intermediate region of the given band) is caused to frequency modulate a high frequency picture carrier over a high frequency deviation range illustratively, 4.3-6.3 MHz). The peak to peak amplitude of the sound modulator output is held at a level which is small relative to the peak-to-peak amplitude level of the picture modulator output, with an illustrative level ratio being 1:10. The respective modulated carriers are combined in a linear adder and applied to a recorder which may be a Halter electromechanical recorder controlled in response to the signal developed by the adder. The recorder is used to record the composite signal as geometric variations (i.e., undulations) on the metal substrate.
The specification of the sound carrier recorded on a video disc is generally critical to the performance of the video disc system. The peak-to-peak amplitude of the sound carrier recorded on a high density information record, such as a video disc described in U.S. Pat. No. 3,842,194 to J. K. Clemens is very small--illustratively, the sound carrier amplitude may be 85 .ANG. peak-to-peak. Deviation of the amplitude of the sound carrier from that which is specified may adversely affect the quality of the video and audio reproduction. For example, if the sound carrier is not cut deep enough the signal-to-noise ratio may be degraded or, on the other hand, if it is cut too deep then sound beats may be visible during the video reproduction.
To assure high quality video and audio reproduction during disc playback, it is generally agreed that certain measurements regarding the quality of the information recorded on the metal substrate should be made prior to producing production line records. The present invention provides an optical playback apparatus that may be used for reproducing, and thus veryifying, the information recorded according to the Halter electromechanical recording method.
In its simplest form, the surface pattern of a metal substrate can be considered as a set of adjacent and parallel one-dimensional gratings with no guard space between the adjacent gratings. The gratings correspond to the signal tracks. The video signal wavelengths, especially on the inner radius signal tracks, are much smaller than the track-to-track spacings. Illustratively, the track spacing is about 2.5 .mu.m.
An objective lens of an optical playback system of numerical aperture (N.A.) whose aperture is fully illuminated with a plane wave of light having wavelength .lambda. produces at its focal plane a focused spot such that about one-half of the optical power is within a circle of diameter D where EQU D=.lambda./2NA (1)
The illuminating optics of the optical playback apparatus should be chosen such that the focused spot diameter is small enough to both resolve the shortest wavelength of interest and to maintain adjacent track crosstalk at an acceptably low level. Therefore, in practice extremely high numerical apertures (e.g., NA&gt;0.8) must be used to resolve the smallest signal wavelengths on the metal substrates.
One optical playback system for reading a metal substrate having signals cut according to a Halter method is described in U.S. Pat. No. 4,065,786 issued on Dec. 27, 1977 to W. C. Stewart. According to the Stewart system, the differential phase representative of the recorded information of a light beam reflected from the metal substrate surface is detected by a split photodetector. Thus, the output signal from the split photodetector is representative of the signal recorded on the metal substrate surface. The frequency response of a differential phase optical playback system to sine wave signals may be approximated by a triangular response characteristic having a peak response in the middle of the frequency band with a linear roll off to an upper and lower cutoff frequency.
Ideally, lenses should be selected such that frequencies of the recorded information occur in the vicinity of the peak response of the optical system. However, it is very difficult to provide an optical system having a uniform response to wideband signals. For example, in a recording system where slot shaped signal elements are recorded on a flat surface, if the differential phase optical readout system is operated with a uniformly illuminated diffraction limited objective lens having a rectangular aperture and the lens is chosen so that the optical readout system is optimized for a video signal of 5 MHz at a particular radius, the response of the system to the 716 KHz audio signal will be about 17 dB lower.
In accordance with U.S. patent application Ser. No. 242,250 entitled "Multi-Bandwidth Optical Playback Apparatus" filed on Mar. 10, 1981 for Istvan Gorog et al., now U.S. Pat. No. 4,375,096 (hereinafter, the Gorog apparatus) an optical playback apparatus is described for reproducing the information recorded on the metal substrate to verify the quality thereof. The Gorog apparatus includes a high numerical aperture lens for use in reading the information recorded. If the high numerical aperture objective lens performed in a truly ideal manner, i.e., as an ideal diffraction limited focusing device, then the reproduction of the information recorded would contain no signal distortions because signal distortions produced in one half of a split detector, as described in the aforementioned Stewart patent, would exactly cancel the signal distortions produced in the other half of the split detector. However, a real objective lens is not ideal and signal distortions that are the result of the imperfections of the lens system and are produced in the square-law detector are present in the reproduced output. The particular type of distortion of interest here is known as baseband distortion. This distortion, that can be produced by an imperfect real optical system when reading video signals encoded according to the Clemens patent, may significantly interfere with the recovery of the audio signals encoded according to the Clemens patent. To reduce these distortions, the Gorog apparatus provides a dual bandwidth apparatus for reading the information. In accordance with the Gorog apparatus the video information, recorded in accordance with the aforementioned Halter patent, is read out using an objective lens having a high numerical aperture while the audio information is readout using an objective lens having a low numerical aperture. The same objective lens is used to readout the video and audio but a stop is interposed in the readout beam path of the audio readout beam. The stop modifies the readout system by effectively reducing the numerical aperture of the objective lens. In other words, the stop acts as a filter for eliminating the video information that, as a result of baseband distortions produced by an imperfect real optical system, may produce interference with the recovery of the audio information.
The Gorog apparatus is quite effective in reducing some of the video baseband distortions which appear in the audio channel, nevertheless, some distortions are still evident and may distract a viewer during verification of a metal substrate. For example, when certain structured scenes, i.e., a picket fence, recorded according to the Halter and Clemens patents on the inner disc radii are being readout with an optical system, using a circularly polarized 633 nm wavelength light beam so much baseband distortion may be produced in the audio channel that the audio information becomes unintelligible.
Baseband distortions in optical readout systems may be thought of as two types. During optical playback of a metal substrate formed in accordance with the aforementioned Clemens format baseband distortion may be produced in an imperfect real optical system by the combination of the aperture response of the detection system and the non-linearity inherent in square-law optical detectors as indicated above. The distortion produced by the aperture and detector responses may be reduced or eliminated by the aforementioned Gorog apparatus. The second source of baseband distortion is the spatial frequency dependent phase shift that is exhibited by light diffracted from a grating wherein the ratio of the wavelength of the readout light beam to the period of the grating approaches, and possibly exceeds, unity. As the grating pitch varies with the content of the video information, phase shifts in the readout light beam used for recovery of the audio information effects signal distortions in the audio channel. This second source of baseband distortion has not been adequately compensated for in prior art devices.